Referring now to FIG. 1, a functional block diagram of a battery pack 100 according to the prior art is presented. The battery pack 100 includes multiple cells 106-1, 106-2, 106-3, and 106-4, each having a positive and a negative terminal. The cells 106 are connected in series, with the positive terminal of one cell 106 connected to the negative terminal of the next cell 106. In various implementations, the cells 106 may be lithium ion (LiIon) charge storage cells.
The negative terminal of the cell 106-4 is connected to an external contact, which may interface with a power tool or a charger. The positive terminal of the cell 106-1 is connected to a switch 110. When the switch 110 is conducting, the positive terminal of the cell 106-1 is connected to an external contact, which may interface with the power tool or the charger. The switch 110 is controlled by a protection system 114. The protection system 114 monitors voltages at the positive and negative terminals of each of the cells 106.
The cells 106 may have varying charge storage capacities. The cells 106 with lower charge storage capacities will decrease in voltage faster than the cells 106 with larger storage capacities. The protection system 114 measures the voltages of the cells 106 and instructs the switch 110 to stop conducting when the voltage of one of the cells becomes too low. In addition, the switch 110 may stop conducting if the current flowing through it exceeds a safe operating level.
Referring now to FIG. 2, a graphical depiction of battery charging according to the prior art is presented. During battery charging, each of the cells is charged to a predetermined voltage. In FIG. 2, four cells are depicted, and they are each charged to 3.6 volts. At 3.6 volts (V), however, the cells may have varying capacities. For example, the first three cells may have a fully charged capacity of 1.1 ampere-hours (Ah), while the fourth cell has a capacity of 1.0 Ah. This imbalance causes problems during discharging, as shown in the next figure.
Referring now to FIG. 3, a graphical depiction of battery discharging according to the prior art is presented. As the battery pack is discharged, the voltages of the cells decrease. After 1.0 Ah has been drawn from the cells, the first three cells have 0.1 Ah remaining, and may be at a voltage of approximately 2.5 V. However, the fourth cell has zero capacity remaining, and may be at a lower voltage, such as 1.8 V, or even a negative voltage.
Battery cells, and especially LiIon cells, may be permanently damaged, and may even become unstable, when driven to low or negative voltages. The voltage of the fourth cell may be too low after 1.0 Ah has been drawn from the battery pack. For this reason, the protection system 114 of FIG. 1 monitors the voltage of each of the cells. Without this monitoring, the user may continue using the battery pack even as one or more of the cells 106 runs out of capacity. As current is drawn from the first three cells, the fourth cell may even be driven to a negative voltage, which is likely detrimental to the fourth cell. The protection system 114 can prevent this from happening, but is expensive and complicated to include in each battery pack.
The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.